Wireless communications equipment typically employs RF amplifiers for increasing RF power during signal transmission. Particular classes of amplification rely on linear amplifiers. Linearity for broadband amplifiers having a bandwidth greater than about 20% is relatively strict.
An indirect measure of linearity is error vector magnitude (EVM), which is typically given in percent or decibels (dB). According to the ETSI TR 290 V1.31 (2014 July), one formula for EVM in percent is:
                              EVM          RMS                =                                                                              1                  N                                ⁢                                                      ∑                                          j                      =                      1                                        N                                    ⁢                                                                          ⁢                                      (                                                                  δ                        ⁢                                                                                                  ⁢                                                  I                          j                          2                                                                    +                                              δ                        ⁢                                                                                                  ⁢                                                  Q                          j                          2                                                                                      )                                                                              S                max                2                                              ×          100          ⁢          %                                    (                  EQ          .                                          ⁢          1                )            Where I and Q are ideal coordinates and, δI and δQ are the errors in the received data points, and N is the number of data points in a measurement sample. Smax is the magnitude for a vector to the outermost state of the constellation.
In this regard, it is desirable to combine two power amplifiers (PAs) to increase linear output power by at least 3 dB without compromising bandwidth or linear efficiency. Additionally, it is further desirable to reduce the EVM percent at the at least 3 dB higher power level.
What is needed is a 3 dB or better linear power amplifier that combines two PAs without compromising bandwidth or linear efficiency versus a single amplifier configuration. Moreover, a need exists for at least a 4 dB EVM linearity improvement across a greater than 20% bandwidth.